docs: def segment_centre_of_mass(seg):
      '''Calculate and return centre of mass of a segment.
  
      C, seg_volalculated as centre of mass of conical frustum'''
      h = mm.segment_length(seg)
      r0 = seg[0][COLS.R]
      r1 = seg[1][COLS.R]
      num = r0 * r0 + 2 * r0 * r1 + 3 * r1 * r1
      denom = 4 * (r0 * r0 + r0 * r1 + r1 * r1)
      centre_of_mass_z_loc = num / denom
      return seg[0][COLS.XYZ] + (centre_of_mass_z_loc / h) * (seg[1][COLS.XYZ] - seg[0][COLS.XYZ])